U.S. patent number 5,431,772 [Application Number 07/963,890] was granted by the patent office on 1995-07-11 for selective silicon nitride plasma etching process.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Wayne T. Babie, Kenneth L. Devries, Bang C. Nguyen, Chau-Hwa J. Yang.
United States Patent |
5,431,772 |
Babie , et al. |
* July 11, 1995 |
**Please see images for:
( Certificate of Correction ) ** |
Selective silicon nitride plasma etching process
Abstract
A two step method of etching a silicon nitride layer carrying a
surface oxygen film from a substrate in a plasma reactor employs
the steps of (1) a breakthrough step of employing a plasma of
oxygen free etchant gases to break through and to remove the
surface oxygen containing film from the surface of the silicon
nitride layer, and (2) a main step of etching the newly exposed
silicon nitride with etchant gases having high selectivity with
respect to the silicon oxide underlying the silicon nitride. The
plasma etching can be performed while employing magnetic-
enhancement of the etching. The plasma etching is performed in a
plasma reactor comprising a low pressure, single wafer tool. Plasma
etching is performed while employing magnetic-enhancement of the
etching. The etchant gases include a halide such as a bromide and a
fluoride in the breakthrough step. The etchant gases include an
oxygen and bromine containing gas in the main step.
Inventors: |
Babie; Wayne T. (Poughkeepsie,
NY), Devries; Kenneth L. (Hopewell Junction, NY), Nguyen;
Bang C. (Wappingers Falls, NY), Yang; Chau-Hwa J.
(Hopewell Junction, NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
[*] Notice: |
The portion of the term of this patent
subsequent to February 23, 2010 has been disclaimed. |
Family
ID: |
24815215 |
Appl.
No.: |
07/963,890 |
Filed: |
October 19, 1992 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
700871 |
May 9, 1991 |
5188704 |
|
|
|
Current U.S.
Class: |
438/714;
257/E21.252; 438/723; 438/724 |
Current CPC
Class: |
H01L
21/31116 (20130101); H01L 21/67069 (20130101) |
Current International
Class: |
H01L
21/02 (20060101); H01L 21/00 (20060101); H01L
21/311 (20060101); B44C 001/22 (); C03C 015/00 ();
C03C 025/06 () |
Field of
Search: |
;156/643,653,657,662,646
;437/238,241 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Breneman; R. Bruce
Assistant Examiner: Everhart; B.
Attorney, Agent or Firm: Balconi-Lamica; Michael J. Jones,
II; Graham S.
Parent Case Text
This is a continuation application of application Ser. No.
07/700,871 filed May 9, 1991, now U.S. Pat. No. 5,188,704.
Claims
What is claimed is:
1. A method of etching a silicon nitride layer on a substrate, said
substrate coated on its surface with a silicon oxide layer, in turn
coated on its surface with a silicon nitride layer, in turn coated
on its surface with an oxide or oxynitride containing film, said
method comprising performing in a plasma reactor the steps on said
substrate, as follows:
(1) a breakthrough step of employing a plasma of oxygen free
etchant gases to break through and to remove said oxide or
oxynitride containing film from said surface of said silicon
nitride layer, and
The gas composition includes as follows:
(2) a main step of etching the newly exposed surface of said
silicon nitride layer with etchant gases having high selectivity
with respect to said silicon oxide layer underlying said silicon
nitride layer employs as follows:
2. A method of etching a silicon nitride layer on a substrate, in
accordance with claim 1 comprising as follows:
(1) Breakthrough step process materials and parameters are employed
as follows:
(2) the main step of etching has parameters as follows:
3. A method of etching a silicon nitride layer on a substrate, in
accordance with claim 2 comprising as follows:
(1) Breakthrough step process materials and parameters are as
follows:
(2) the main step of etching has parameters as follows:
4. A method of etching a silicon nitride layer on a substrate, said
substrate coated on its surface with a silicon oxide layer, in turn
coated on its surface with a silicon nitride layer, in turn coated
on its surface with an oxide or oxynitride containing film, said
method comprising performing in a plasma reactor the steps, as
follows:
(1) a breakthrough step of employing a plasma of oxygen free
etchant gases to break through and to remove said oxide or
oxynitride containing film from said surface of said silicon
nitride layer, under the conditions as follows:
(2) a main step of etching the newly exposed surface of said
silicon nitride layer with etchant gases having high selectivity
with respect to said silicon oxide layer underlying said silicon
nitride layer employs as follows:
5. A method of etching a silicon nitride layer on a substrate, in
accordance with claim 4 comprising as follows:
(1) Breakthrough step process materials and parameters are employed
as follows:
(2) the main step of etching has parameters as follows:
6. A method of etching a silicon nitride layer on a substrate, in
accordance with claim 5 comprising as follows:
(1) Breakthrough step process materials and parameters are employed
as follows:
(2) the main step of etching has parameters as follows:
7. A method of etching a silicon nitride layer on a substrate, said
substrate coated on its surface with a silicon oxide layer, in turn
coated on its surface with a silicon nitride layer, in turn coated
on its surface with an oxide or oxynitride containing film, said
method comprising performing in a plasma reactor the steps on said
substrate, as follows:
(1) a breakthrough step of employing a plasma of oxygen free
etchant gases to break through and to remove said oxide or
oxynitride containing film from said surface of said silicon
nitride layer, and
The gas composition includes
(2) a main step of etching the newly exposed surface of said
silicon nitride layer with etchant gases having high selectivity
with respect to said silicon oxide layer underlying said silicon
nitride layer employs as follows:
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to plasma etching of silicon nitride.
2. Discussion of Related Art
One selective nitride etching process for producing an sidewall
process utilizes a Cl.sub.2 /O.sub.2 /Ar plasma etching chemistry
at a selectivity of nitride over oxide of about 4-5:1. Overetching
of such a silicon nitride layer can lead to total loss of an
underlying thin SiO.sub.2 stopping layer and there is the potential
of damage to the underlying Si substrate at the surface juxtaposed
with the emitter opening in the surface of the Si substrate of a
transistor being formed in the Si substrate.
U.S. Pat. No. 4,374,698 of Sanders et al, for "Method of
Manufacturing a Semiconductor Device" etches silicon nitride and
silicon oxide in a plasma with the silicon nitride layers being
etched five times faster than the silicon oxide layers. The use of
a gaseous compound containing a halogen other than a fluoride is
described.
The gas composition preferably includes
______________________________________ 62.5% SiF.sub.4 A halogen
compound other than a fluoride: 7.5% (2-8% pref) CF.sub.3 Br or
1-15% CF.sub.2 Cl2 An oxidation compound or molecule: 30% NO (range
20-40%) or 3-10% O.sub.2 0% diluent gas listed Preferred reactor
conditions pressure 100 Pascal temperature 125 deg. C. substrate
temp. RF power 150 Watts ______________________________________
U.S. Pat. No. 4,793,897 of Dunfield et al for "Selective Film Etch
Process" uses " . . . a reactant gas mixture of fluorinated etching
gas and oxygen for selectively etching a thin film of material such
as silicon nitride with high selectivity for a silicon oxide
underlayer . . . " without any magnetic field and without any other
halides than SiF.sub.4.
The gas composition preferably includes
______________________________________ 10-400 sccm Total gas flow
0-100 sccm NF.sub.3 0-100 sccm SiF.sub.4 0-100 sccm O.sub.2 0-100
sccm He 10-150 sccm Chamber Preferred reactor conditions pressure
0.5-30 mTorr temperature 25 degrees C. RF power 100-1500 watts RF
Frequency none listed ______________________________________
U.S. Pat. No. 4,844,773 of Loewenstein et al for "Process for
Etchings Silicon Nitride Film" with respect to FIG. 32 and the
process unit 1300 describes a process adapted for a low pressure
silicon nitride etch. It is stated that "HBr or CF.sub.3 Br
provides a very potent passivating chemistry for fluorine-based
etches." The Loewenstein patent suggests use of SiF.sub.4 with HBr
to etch a thin film of tungsten. However, with respect to silicon
nitride, the examples of etches are as follows:
EXAMPLE 1
Gas composition preferably includes
______________________________________ 1000 sccm Helium 200 sccm
CF.sub.4 (F.sub.2, CHF.sub.3, C2F.sub.6, SF.sub.6, F.sub.3, or
combinations with CF.sub.4) Preferred reactor conditions pressure
0.7 Torr temperature 25 degrees C. RF power 225 watts RF Frequency
13.56 MHz Remote RF power 400 watts Remote RF Frequency 2450.00 MHz
______________________________________
EXAMPLE 2
Gas composition preferably includes
______________________________________ 500 sccm Helium 100 sccm
SF.sub.6 (or F.sub.2, CF.sub.4, or C2F.sub.6) Preferred reactor
conditions pressure 50 mTorr, temperature 25 degrees C. RF power
200-300 watts RF Frequency 13.56 MHz Remote RF power 400 watts
Remote RF Frequency 2450.00 MHz
______________________________________
The Loewenstein et al patent mentions a magnetron shown in FIG. 15
of that patent in connection with a remote plasma chamber as "an
example of a structure which generates activated species by gas
flows through a plasma discharge which is remote from the wafer
face . . . " There is no mention of the application of a magnetic
field to a wafer directly. Instead, a remote gas plasma mixture is
applied to the work. There is no discussion of a magnetically
confined plasma, but only the remote magnetron is discussed.
It, is an object of this invention to provide a new etching
process, capable of etching silicon nitride over SiO.sub.2 very
selectively for products where the semiconductor device structure
would have an even thinner SiO.sub.2 stopping layer during silicon
nitride etching where it is required to assure product
manufacturability and acceptable yield capacity in
manufacturing.
In accordance with-this invention silicon nitride is etched with a
selectivity with respect to SiO.sub.2 better than 6:1 up to over
100:1 depending upon the gas composition and the process conditions
employed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B show a chamber housing a plasma etching reactor
with an electromagnetic unit for enhancing the plasma.
FIG. 2 and FIG. 3 are graphs of etch rates and nitride to oxide
selectivity respectively as a function flow rates of helium and
oxygen.
FIG. 4 is a graph of etch rate as a function of flow rates of
helium and oxygen with SiF.sub.4 addition.
FIG. 5 is a graph of nitride/oxide selectivity as a function of
helium oxygen flow rates with SiF.sub.4 addition.
FIG. 6 is a sectional view of a substrate having an opening with a
layer of silicon dioxide at the base. The substrate is prepared for
processing with silicon nitride.
FIG. 7 is a view of the product of FIG. 6 after a layer of silicon
nitride has been deposited.
FIG. 8 is a view of the product of FIG. 7 after a layer of silicon
dioxide has been deposited on the layer of silicon nitride.
FIG. 9 is a view of the product of FIG. 8 after the top layer of
silicon dioxide has been etched away leaving a sidewall structure
of silicon dioxide on the sides of the opening in the
substrate.
FIG. 10 is a view of the product of FIG. 9 after the silicon
nitride exposed has been etched away between the sidewall
structures.
DESCRIPTION OF THE PREFERRED EMBODIMENT
1. Process Overview
This invention utilizes a two-step process in a low pressure,
single wafer, plasma reactor employing magnetic enhancement to
perform the steps as follows:
(1) break through and to remove surface oxide or oxynitride from
the surface of a top silicon nitride layer, and
(2) to etch the newly exposed silicon nitride with high selectivity
with respect to the silicon oxide underlying the silicon
nitride.
The first step employs fluorine radicals to etch away the surface
oxygen impurities from the silicon nitride layer in an oxygen-free
plasma. Then, the second step uses bromine-containing and
oxygen-containing gases to etch primarily the exposed surfaces of
the silicon nitride layer in a plasma etching atmosphere which is
also magnetically enhanced.
The preferred reactive gas for the step of breaking through the
oxynitride layer is SF.sub.6 gas with or without a small amount of
HBr gas as an additive. The silicon nitride surface oxide or
oxynitride impurities, normally less than 50 Angstroms in thickness
can be removed from the work piece in a gaseous plasma within a few
seconds. The preferred main etchant for silicon nitride etching is
a mixture of HBr and O.sub.2, with or without diluent such as He.
HBr dissociates readily in the presence of oxygen in the plasma and
its dissociation can be enhanced by the application of a rotational
magnetic field. (See reaction equation (1) below.) The bromine
radical is the main etchant for silicon nitride. (See reaction
equation (2) below.) At the point of depletion of silicon nitride,
an equilibrium exists at the nitride-oxide interface where the rate
of oxide etching (forward reaction) is competing with the
re-oxidation of silicon bromide or redeposition of oxide (reversed
reaction, (see reaction equation (3) below). This equilibrium
results in a substantially lower rate of etching of the underlying
oxide which favors a high selectivity of silicon nitride-to-silicon
oxide in the overall reaction. ##STR1##
Silicon-containing gases such a silicon tetrafluoride can be used
as an additive agent to increase the deposition of silicon oxide at
the substrate surface, thus further depressing the etching of
silicon oxide in reaction (3) and resulting in an even higher
degree of selectivity. However, the addition of silicon
tetrafluoride also reduces the etching rate of silicon nitride,
which can have a negative impact upon wafer processing throughput
in semiconductor device manufacturing.
Plasma Processing System
FIGS. 1A and 1B show a plasma processing apparatus suitable for use
for performing the processes of this invention. A plasma reactor 20
includes walls 21 housing a reactor chamber 22 wherein a plasma of
neutral (n) particles, positive (+) paticles, and negative (-)
particles are found. Walls 21 include cylindrical wall 44 and cover
46. Plasma processing gases are introduced via inlets 24 into
reactor chamber 22. Plasma etching gases are introduced into
chamber 22 through inlets 24. A water cooled cathode 26 is
connected to an RF power supply, 28 at 13.56 MHz. An anode 29 is
connected to the walls 21 which are grounded by line 30. Helium gas
is supplied through passageway 40 through cathode 26 to the space
beneath wafer 38 which is supported peripherally by lip seal ring
42 so that the helium gas cools the wafer 38. Wafer support 36
includes a plurality of clamps not shown which hold down the upper
surface of wafer 38 at its periphery as is well known to those
skilled in the art. A pair of helmholtz configured electromagnetic
coils 32 and 33 providing north and south poles within the chamber
22 are disposed at opposite ends of the lateral cylindrical wall 44
of the walls 21. The coils 32 and 33 provide a transverse magnetic
field with the north and south poles at the left and right
providing a horizontal magnetic field axis parallel to the surface
of the wafer 38. The transverse magnetic field is applied to slow
the vertical velocity of the electrons which are accelerated
radially by the magnetic field as they move towards the wafer.
Accordingly, the quantity of electrons in the plasma is increased
by means of the transverse magnetic field and the plasma is
enhanced as described in Foster et al U.S. Pat. No. 4,668,365 for
"Magnetron Enhanced Plasma Etching Process", Wong et al European
Patent Application No. EP0 272 143-A for "Bromine and Iodine Etch
Process for Silicon and Silicides", and Andrews et al European
Patent Application No. EP-0 272 142-A which relates to a
magnetically enhanced plasma etch reactor for semiconductor wafers
which provides a magnetically controlled magnetic field for
etching. Electromagnets which provide the magnetic field are
independently controlled to produce a field intensity orientation
which is uniform. The field can be stepped angularly around the
wafer by rotating the energization of the electromagnets,
sequentially. U.S. Pat. No. 4,740,268 of Bukhman describes another
"Magnetically Enhanced Plasma System" which rotates a transverse
magnetic flux field B over a wafer, parallel to its surface, in an
electrically excited plasma chamber. The lines of flux are normal
to the electric field E which is directed normal to the wafer of
the lower electrode in the Bukhman patent.
2. Magnetic Field Enhancement
The use of a transverse magnetic field directed parallel to the
surface being treated by the plasma and the cathode of the plasma
reactor increases ionization efficiency of the electrons in the
plasma. This provides the ability to decrease the potential drop
across the cathode sheath and to increase the ion current flux
present on the wafer surface, thereby permitting higher rates of
etching without requiring higher ion energies to achieve the result
otherwise.
The preferred magnetic source used to achieve magnetically enhanced
RIE used in practicing this invention is a variable rotational
field provided by two sets of electromagnetic coils arranged in a
Helmholtz configuration. The coils are driven by 3-phase AC
currents. The magnetic field with flux B is parallel to the wafer
substrate, and perpendicular to the electrical field as shown in
FIG. 1A. Referring to FIG. 1B, the vector of the magnetic field H
which produces flux B is rotating around the center axis of the
electrical field by varying the phases of current flowing through
coils at a typical rotational frequency of 0.01 to 1 Hz,
particularly at 0.5 Hz. The variable strength of the magnetic flux
B typically from 0-150 Gauss is determined by the quantities of the
currents supplied to the coils.
In the case of etching of silicon nitride with an etchant of gases
including bromine and oxygen, the magnetic field flux, typically at
30-60 Gauss provides enhancement to the dissociation of the Br
compounds (reaction (1)) and also O.sub.2 gas, and thus increases
the rate of etching of silicon nitride. On the other hand, the
etching-reoxidation equilibrium of reaction (3) is not being
affected. Thus, the higher selectivity of silicon nitride to the
underlying silicon oxide is obtained.
3. Process Applications and Trends
Highly selective etching of silicon nitride over underlying silicon
oxide can be achieved by taking advantage of the magnetically
enhanced plasma in the reactor system of FIG. 1A using gases
containing bromine and oxygen. The flow rates of oxidant etchant
such as O.sub.2 control the rate of etching and the selectivity of
nitride/oxide as shown in FIGS. 2 and 3. At these selected
conditions, nitride etching rates are not a strong function of the
percent of oxygen in the total gas flow. However, the oxide etching
rate is initially decreased as small amounts of-oxygen are included
in the gas mixture, and the oxide etching rate levels off as the
oxygen flow rate is increased further. The helium in the etchant
gases is used as a reaction inert material or as a diluent, which
does not affect the etching rate or the selectivity of the etching
process. The different process trends for nitride and oxide rates
of etching with respect to increasing oxygen composition in the
total gas flow allow the selectivity of nitride to oxide to
increase an to reach an optimum at around 12.6:1, as indicated in
FIG. 3. Referring to reaction equation (3) above, increasing the
flow rate of oxygen gas can result in an increase in the rate of
re-oxidation of SiBr.sub.y species and can form deposited SiOx
.sup.- r SiO.sub.2, which can cause a reduction of the etching rate
which can cause a corresponding increase of relative etching
selectivity of nitride-to-oxide. However, a further increase of the
flow rate of oxygen gas can result in excess SiO.sub.2 formation
which hampers the rate of etching of the nitride. In general, an
optimal rate of etching and an optimal selectivity ratio are both
dependent upon an appropriate composition of the O.sub.2 etchant in
the total gas flowing through the chamber.
FIG. 4 indicates the effect of adding a silicon containing gas,
particularly SiF.sub.4, to the reactive gas. Addition of SiF.sub.4
can have an effect on rate of etching for selected process
conditions. As indicated by FIG. 4, both the rates of nitride and
oxide etching were more significantly influenced by the increase of
the oxygen flow rate in the presence of 1.5 sccm (standard cubic cm
per minute) of SiF.sub.4 in the total gas flow. It is envisioned as
the oxygen concentration increases that the rake of SiO.sub.2
deposition increases sharply as a result of the following reaction:
##STR2## Both nitride and oxide rates of etching have shown a
downward trend as the SiO.sub.2 deposition increases with the
increasing of the oxygen flow rate. The nitride-to-oxide
selectivity is calculated from the nitride etching rate divided by
the oxide etching rate, which increases sharply with increasing
oxygen flow and it reaches infinity at a zero oxide rate as shown
in FIG. 5. Further increase in the oxygen flow rate results in a
net SiO.sub.2 deposition at the nitride/oxide interface.
FIG. 1A AND FIG. 1B shows a chamber housing a plasma etching
reactor with an electromagnetic unit for enhancing the plasma.
Method 1
We have discovered when etching away films of silicon nitride with
fluoride containing etchants in the presence of oxygen that a film
of thin oxide or oxynitride prevents rapid etching of the silicon
nitride film. However, the use of oxygen is helpful for the purpose
of etching away the silicon nitride once the oxynitride film has
been removed from the surface of the silicon nitride layer.
Accordingly we have developed a two step process of plasma etching
silicon nitride selectively with respect to other films such as
silicon oxide or silicon dioxide.
PROCESS I
Breakthrough of the Surface of Oxynitride
The first phase of the process is to break through, i.e. clean and
etch away the surface oxynitride which is a thin layer of about 50
Angstroms in thickness on the surface of the layer of silicon
nitride. The first phase is performed in the absence of an oxidant
such as oxygen, since the purpose of this step is to reduce the
oxygen which forms the oxynitride layer. In the case of both
Process I and Process II below, a reactive plasma is formed in the
reactor including HBr and a gas from the group of
fluorine-containing gases consisting essentially of SF.sub.6,
CF.sub.4 and C2F.sub.6 and NF.sub.3. The gas composition preferably
includes
______________________________________ (0-90%) HBr (10-100%)
fluorine-containing gas. (0%) oxygen or oxidant gas. Preferred
reactor conditions pressure low Temperature 25 degrees C. RF power
200-300 watts rotational magnetic field 0-45 Gauss.
______________________________________
Main Etching Step: Plasma Etching of Si.sub.3 N.sub.4
In the second phase of the process, with the oxynitride removed,
the exposed layer of Si.sub.3 N.sub.4 is etched with a bromine and
oxygen containing reactive gas mixture preferably consisting of
HBr, an oxidant selected from the group consisting of O.sub.2,
CO.sub.2, or N.sub.2 O and a diluent gas such as He, N.sub.2 or Ar.
It will be noted that in this case the main etching step is
performed in the absence of SiF.sub.4 or any other fluorine
containing gases. The preferred reactor conditions are as
follows:
TABLE I
The gas composition broadly includes as follows:
______________________________________ 60-95% HBr 0% SiF.sub.4
2-15% an oxidant (O.sub.2, CO.sub.2, or N.sub.2 O) 0-45% diluent
gas Preferred reactor conditions pressure 50-150 mTorr RF power
200-400 watts rotational magnetic field 25-70 Gauss Si.sub.3
N.sub.4 etch rate 300-1000 Angstroms/min
______________________________________
The selectivity of Si.sub.3 N.sub.4 /SiO.sub.2 is better than 6:1
up to 12.6:1 in our experiments.
More generally, the process parameters fall into ranges as follows
listed on the basis of flow rates of the gases as listed in TABLE
II below.
TABLE II ______________________________________ Process Broad
Preferred Optimum ______________________________________ Gas Flow,
sccm HBr 0-20 (0-91%) 5-15 (45-83%) 10 (77%) SF.sub.6 2-10 (9-100%)
3-6 (17-55%) 3 (23%) Pressure, mT 20-150 50-100 50 Power Density
0.82-3.3 1.63-2.4 1.65 (W/cm2) Time, sec 4-20 6-10 8 Magnetic Field
0-45 0-25 0 Gauss MAIN ETCH Gas Flow, sccm HBr 10-30 (71-96% 15-25
(80-95%) 20.00 (93%) O.sub.2 0.15-2 (2-7%) 0.3-1 (2-5.7%) 0.45
(2.1%) He 0-10 (0-25%) 0-2 (0-20%) 1.05 (4.9%) Pressure, mT 20-150
50-120 100 Power Density 0.82-3.3 1.63-2.4 1.65 (W/cm2) Etch Rate
300-1000 500-700 600 (A/min) Magnetic Field 0-75 25-60 45 Gauss
Si.sub.3 N.sub.4 -to-SiO.sub.2 selectivity LPCVD Nitride >4:1
>6:1 12.6:1 Plasma Nitride >4:1 >6:1 13.5:
______________________________________
PROCESS II
Plasma etching the Si.sub.3 N.sub.4 layer over the SiO.sub.2 layer
in a magnetically-enhanced etch reactor with the steps as
follows.
Breakthrough of the Surface of Oxynitride
This step is the same as in proposed process I.
Main Etching Step: Plasma Etching of Si.sub.3 N.sub.4
In the second phase of process II, with the oxynitride removed, the
now exposed layer Si.sub.3 N.sub.4 is etched with a bromine, oxygen
and fluorine containing reactive gas mixture preferably consisting
of HBr, SiF.sub.4, an oxidant selected from the group consisting of
O.sub.2, CO.sub.2, or N.sub.2 O and a diluent gas selected from the
group He, N.sub.2 and Ar. The preferred reactor conditions are as
follows:
The gas composition broadly can include as follows:
______________________________________ 50-83% HBr 5-15% SiF.sub.4
2-15% an oxidant (O.sub.2, CO.sub.2, or N.sub.2 O) 0-45% diluent
gas Preferred reactor conditions pressure 50-150 mTorr, RF power
200-400 watts rotational magnetic field 25-70 Gauss. Si.sub.3
N.sub.4 etch rate 300-800 A/min
______________________________________
Using the process II etching of nitride occurs with a selectivity
of Si.sub.3 N.sub.4 /SiO.sub.2 of better than 6:1 and up to over
100:1. However, a loss of nitride etch rate an an increase in etch
non-uniformity are accompanied with the increased selectivity.
The data listed in TABLE IV below illustrates the useful ranges of
process parameters for selectively etching the nitride over
underlying thermal oxide or plasma oxide. The selectivity for the
plasma deposited nitride is slightly higher than that for LPCVD
(Low Pressure Chemical Vapor Deposition) nitride. The application
of magnetic field enhancement to the plasma is essential for
providing high ionization efficiency and adequate etching rate of
the nitride layer. Also as shown in Table II, the addition of
SiF.sub.4 in the HBr/O.sub.2 /He etchant is the main etching step
which enhances the rate of SiO.sub.2 deposition and increases
nitride to-oxide selectivity.
TABLE IV ______________________________________ Process Broad
Preferred Optimum ______________________________________ Gas Flow,
sccm HBr 0-20 5-15 10 SF.sub.6 2-10 3-6 3 Pressure, mT 20-150
50-100 50 Power Density 0.82-3.3 1.63-2.4 1.65 (W/cm2) Time, sec
4-20 6-10 8 Magnetic Field 0-45 0-25 0 Gauss MAIN ETCH Gas Flow,
sccm HBr 10-30 (50-90%) 10-20 (70-85%) 15 (83%) O.sub.2 0.15-2
(2-15%) 0.3-1 (3-7%) .45 (2.6%) He 0-10 (0-30%) 0-4 (0-20%) 1.05
(6.0%) SiF.sub.4 0.5-2.5 (5-15%) 1-2 (5-9%) 1.5 (8.4%) Pressure, mT
20-150 50-120 100 Power Density 0.82-3.3 1.63-2.4 1.65 (W/cm2) Etch
Rate 200-700 250-450 300 (A/min) Magnetic Field 0-75 25-60 45 Gauss
Si.sub.3 N.sub.4 -to- >6:1 >10:1 17:1 SiO.sub.2 selectivity
LPCVD Nitride ______________________________________
EXAMPLE
Referring to FIGS. 6-10, the drawings show sectional views of the
results of a sequence of steps in an integrated advanced bipolar
semiconductor sidewall process. Referring to FIG. 6, a wafer with a
silicon substrate 10, is coated with a film 11 of polysilicon which
has a contact opening 12 with a lower layer of SiO.sub.2 14 at the
bottom of opening 12. The process includes steps as follows:
1) FIG. 7 shows a deposit on the wafer of FIG. 6 of a thin film 16
of Si.sub.3 N.sub.4 (approx. 200-500 Angstroms thick) over the film
11 and over the layer of SiO.sub.2 14 at the base of opening 12.
The opening 12 and sidewall and the polysilicon film 11 are all
coated with the silicon nitride film 16.
2) Referring to FIG. 8, using a TEOS (TetraEthylOrthoSilicate)
liquid, deposit SiO.sub.2 (approx. 1000-3000 Angstroms thick) on
top of the nitride layer produced in step 1) above. This produces
an SiO.sub.2 sidewall layer 18 on top of thin film 16.
3) Referring to FIG. 9, an etch of the top SiO.sub.2 layer 18 has
been performed to leave just the sidewalls 18' as the lower layer
of Si.sub.3 N.sub.4 16 is exposed by the removal of the remainder
of layer 18. A CF.sub.4 /Ar plasma forms the SiO.sub.2 sidewalls
18' at the opening with a 15% overetch. The endpoint is controlled
by an optical emission monitoring device.
4) In FIG. 10 is shown the device of FIG. 9 after etching away of
the exposed surface of the underlying Si.sub.3 N.sub.4 layer 16 in
hole 12, exposing the surface of layer 14 of SiO.sub.2 from FIG. 6,
which is carrying the sidewall pattern 18', as well as, the edges
of layer 16. The etchant used is HBr/SF.sub.6 and HBr/O.sub.2 /He
according to the process conditions in Process I above. The process
I steps is performed by a time-etch of 60 sec. which includes a 25%
overetch. Measurements have indicated a complete removal of nitride
over SiO.sub.2 and no measurable loss of the SiO.sub.2 in the
sidewall pattern 18' during that etch.
* * * * *